Typically, a wafer comprising a plurality of semiconductor dice are mounted on an adhesive film during singulation wherein each individual die is separated while it adheres to the adhesive film. Detaching a die from an adhesive film is thus a common process involved in die bonding and flip chip bonding processes for assembling an electronic package. One trend for the development of high density electronic devices is to multiply its density on the same footprint by stacking up the dice comprised in an electronic package. The thickness of each die being stacked in the package has to be reduced in order to minimize the final height of the package. It becomes a challenging task to detach a die from an adhesive film without damaging the die when the thickness of the die is reduced to below 4 mils (about 100 microns). Dice with thickness of 3-4 mils (75-100 microns) have been used for mass production for some time. Mass production of dice at 2-3 mils (50-75 microns) thick is currently under preparation. Experiments for research and development in electronic packaging designs are ongoing for dice of 1-2 mils (25-50 microns) thick. Hence, an apparatus that is capable of reliably detaching very thin dice from adhesive films is becoming a critical component in electronic assembly equipment.
There are various forms of ejector pin designs for facilitating detachment of a die from an adhesive film to which it is mounted. The simplest is a single ejector pin design, which is a traditional design for detaching a small die from an adhesive film. As the size of the die gets bigger (say, more than 4×4 mm), multiple ejector pins may instead be used for detaching the die from the adhesive film.
In U.S. Pat. No. 6,386,815 entitled “Pick-Up Apparatus for Semiconductor Chips”, a multiple ejector pins design is disclosed comprising an integrated or a separately rotatable composite cam formed by a plurality of cams and a plurality of pin holders having a plurality of ejector pins, wherein at least a part of the pin holder is in contact with the composite cam. Two sets of multiple ejector pins are mounted on the same set of holders that are capable of providing different pin arrangement configurations. This aids in avoiding conversion time that is otherwise necessary for setting up different pin arrangements as the same pin holders capable of providing two sets of pin arrangements for handling dice of different sizes can be used for detaching dice from the adhesive film. The same pin holders can be used for dice of various sizes in accordance with those pin arrangements without replacing the pin holder.
Another design is disclosed in U.S. Pat. Nos. 4,850,780 and 4,990,051 for a “Pre-Peel Die Ejector Apparatus”, wherein a single ejector pin is incorporated with a die eject collar around the ejector pin. The pre-peeling collar is first moved to stretch the adhesive tape to pull it away from the die to be detached, and the die is subsequently further separated from the adhesive tape by the single ejector pin. This design can be used to detach somewhat larger dice from adhesive films.
Yet another approach is described in U.S. Pat. No. 6,201,306 entitled “Push-Up Pin of a Semiconductor Element Pushing-Up Device, and a Method for Separating”. This approach utilizes multiple ejector pins with their tips modified to spherical shapes, in order to reduce localized stress acting on the die by the ejector pins during the push-up action.
As can be observed from the above prior art examples, conventional multiple ejector pin assemblies may comprise one or more ejector pins mounted on a pin holder having one or more stages. The target die attached on an adhesive film will be positioned to the center of a vacuum enclosure which contains the ejector pin assembly. Before the ejector pins move up to push against the die through the deformable adhesive film, vacuum suction will be applied within the vacuum enclosure. The suction will hold the adhesive film down onto the top platform of the vacuum enclosure via the vacuum holes on the platform. The adhesive film carrying the die lies flat on top of the platform under compression by atmospheric pressure. The ejector pins will then elevate from below and rise above the top surface of the platform, so that the pins will push the die up and induce a bending moment on the die in order to separate the adhesive film from the die.
The localized stress from the ejector pins acting on the die can be quite high. The push-up action of the ejector pins will induce relatively large deformations on the die at the locations right above the pins. This may cause die crack failure if the induced strain exceeds its critical value. On the other hand, the bending moment on the die will induce a peeling stress along the interface between the die and the adhesive film. This peeling stress will be increased with the bending moment as the ejector pins are pushed further up. If the peeling stress is large enough to overcome the adhesion between the die and the adhesive film, the die will eventually detach from the adhesive film.
The multiple ejector pins are usually arranged in such a way that they are distributed evenly covering most of the die so that the induced stress can be distributed evenly. The number of pins should be optimized so that the die will be almost totally detached from the adhesive film with the exception of residual contact areas where the pins are supporting the die at the end of the push-up motion of the pins. If the number of pins is increased or pins with spherical tips are used, the total contact area will be large and it will jeopardize the successfulness of the die pick-up process. This contact area can be reduced if fewer ejector pins and/or ejector pins with smaller pin tip radii are used. If the radius of the pin tip is smaller, then the stress on the bottom surface of the die will be increased and the induced strain on the top surface of the die at the location right above the ejector pin will also be increased. The bending moment and the induced strain on the top surface of the die will be correspondingly increased if the thickness of the die is reduced.
Therefore, a shortcoming of prior art ejector pin assemblies is that compromises must often be made between the number of ejector pins and the geometry of the pin tips being used in order to minimize the contact area at the end of the push-up motion of the pins, and to even out the induced stress on the die during the push-up motion of the pins. Nevertheless, such compromise may not be easily achievable if the thickness of the die is decreased to less than 4 mils or if the adhesive strength of the adhesive film is larger than 20 J/m2.